The disclosure relates to vapor compression systems. More particularly, the disclosure relates to such systems having electric motor-driven compressors.
One particular use of electric motor-driven compressors is liquid chillers. An exemplary liquid chiller uses a semi-hermetic centrifugal compressor. The exemplary unit comprises a standalone combination of the compressor, a condenser unit, an evaporator unit, an expansion device, and various additional components. Some such exemplary compressors include a transmission intervening between the motor rotor and the impeller to drive the impeller at a faster speed than the motor.
In various compressors, the motor may be exposed to a bypass of refrigerant flow to cool the motor and/or lubricate bearings.
In most refrigeration systems (especially those using screw compressors and reciprocating compressors), a lubricant (e.g., oil) is added to the refrigerant. The oil may be selectively separated from the refrigerant flow and reintroduced for lubrication (e.g., separated in a mechanical separator or still and then returned to lubrication ports along the bearings. Other compressors (especially centrifugal compressors) are oil-free. In such oil-free compressors, refrigerant itself may be directed to the bearings to cool and lubricate the bearings. Exemplary bearings are ball bearing-type bearings where the balls are made from ceramic materials. The refrigerant may be drawn by a mechanical pump for delivery to the bearings.
In such oil-free compressors, providing startup lubrication has posed problems. Depending upon operational conditions, the inlet port of a mechanical pump may be non-advantageously positioned to provide refrigerant. U.S. Pat. No. 6,654,560 discloses a dual-impeller pump wherein one impeller is positioned to draw from the evaporator and another impeller is positioned to draw from the condenser.